An optical device using a conventional MEMS (below, “optical MEMS”) is shown in FIG. 1 (Fujita Hiroyuki, “Micro Nanomachine Technology Primer”, p. 174 (Kogyo Chosaikai Publishing, Aug. 15, 2003)). The illustrated reflection mirror device 10 is a reflection mirror layer L1 provided with a reflection mirror 12, a tiltable layer L2 having a tilt plate 18 tiltably joined to a base 14 through a torsion spring 16, a fixed layer L3 including a base 22 and bottom electrode 24, and a layer L4 of a CMOS memory, that is, a total of four layers. When the mirror 12 of the layer L1 engages with the tilt plate 18 of the layer L2 and voltage is applied between the tilt plate 18 of the layer L2 and the bottom electrode 24 of the layer L3, the Coulomb force acting between the tilt plate 18/bottom electrode 24 causes the tilt plate 18 of the layer L2 to tilt and the reflection mirror 12 engaged with the tilt plate 18 to tilt together with it. The CMOS memory of the layer L4 stores the operating state due to the above tilt. When the application of voltage between the tilt plate 18/bottom electrode 24 is stopped, the elasticity of the torsion spring 16 causes the mirror 12 to return to the posture before tilt together with the tilt plate 18.
In the optical MEMS 10 serving as this reflection mirror device, for projector use (image projection use), one mirror 12 forms one pixel. The mirror 12 has a size of 25 μm×25 μm or so or remarkably larger compared with the size of patterns of an LSI (less than several μm). Corresponding to the mirror size, the structure of the optical MEMS 10 as a whole also becomes large. The result is a complicated structure where a large number of members are engaged with each other over four layers. The production process also becomes troublesome.
On top of this, the torsion spring enabling tilt/return of the mirror 12 is made of metal which becomes fatigued and easily breaks upon repeated torsional deformation, so is low in reliability.
The operating speed of the mirror 12 need only be one enabling video viewed by a person to be displayed, so is a slow 1 kHz to 1 MHz or so. In particular, for use as a modulator for high speed communications etc., operation at several 10 MHz to several GHz is necessary. Compared with this, the speed is 1 to 6 orders slower. An optical MEMS is promising not as a video device, but as an optical modulator for optical communications between chips or between boards. The operating frequency of the optical MEMS is determined by the resonance frequency of the structure, so to increase the operating speed, it is necessary to make the size smaller to increase the resonance frequency. To make the size smaller, the structure has to be made simpler and production facilitated. Further, as the operating frequency becomes higher, the number of times of repeated operations also remarkably increases, so it is necessary to improve the fatigue strength to increase the reliability.
As another application of an MEMS, application to a switch may also be expected. In recent years, usage of high frequencies such as by mobile phones, the Internet, Bluetooth, etc. has remarkably grown. The conventional frequency bands are becoming overly congested. Therefore, there has been a shift to the higher frequency regions. With high frequency circuits or RF circuits, the interference between circuits (parasitic capacitance and parasitic resistance) has to be eliminated as much as possible, so current transistors which only switch between a low resistance state and high resistance state between ON/OFF are insufficient. Rather, the old “switches” which completely cut or connected circuits—which were most prevalent before the appearance of transistors—are desirable since they can reduce the parasitic capacitance or parasitic resistance down to the ideal state.
However, as such a switch, mounting together with an LSI enabling complicated circuits to be realized at a high density is demanded. Up until now, there have been MEMS's mounted together with LSIs, but these have been large in size, low in operating speed, and large in power consumption, so complete integration with LSI's has not been achieved. Therefore, ultramicro switches able to be completely mounted together with LSI's have been desired.
As still another application of MEMS's, application to memories is promising. With memories, the increasing definition and speed of image processing has led to demands for new development of more inexpensive, larger capacity nonvolatile memories.
As another application of MEMS's, sensors are also promising. To realize greater safety, security, and comfort in all sorts of areas of human life such as electrical products, automobiles, and robots, various types of sensors are spreading in use. As a future trend, the types and numbers of sensors utilized are also expected to increase. It is expected that “smart sensors” able to immediately make complicated judgments by combination with LSIs will become the mainstream. A large variety of sensors can be selectively produced by a single production process. Development of extremely fine sensors able to be fully mounted with LSI's is therefore desired.
In addition, as modes of application of MEMS able to be integrally mounted with LSIs, variable capacity capacitors, variable direction directional antennas, drive devices (motors), power transmission mechanisms, etc. may also be expected.
Most conventional MEMS's are being produced by processing Si semiconductor monocrystalline substrates. Competition arises for substrate space with the LSI circuits fabricated in the same monocrystalline substrate, so the degree of integration has been hard to improve. Further, the process of production of MEMS's and the process of production of LSIs interfere with each other in temperature and formation of step differences, so the performances of the two deteriorate.
In particular, in LSI devices mounting conventional MEMS's, to secure the mechanical strength of moving parts of MEMS's, since the parts are built into the monocrystalline semiconductor substrate, production of the MEMS's has also had to be started from the initial stage of the LSI production process. As a result, MEMS's produced at parts of semiconductor substrates have been exposed to the high temperature heat treatments required for LSI production and have unavoidably deteriorated in characteristics.
To avoid this, it has been necessary to change the process of production of LSIs so that such interference does not occur. The precious library of LSI production technology built up through long experience cannot be effectively utilized and costs rise due to the expenses for development of new processes.
Ideally, if it were possible to form various MEMS's suitable for the above various applications by a low temperature process on a semiconductor substrate on which LSI circuits were completed without having any effect on the LSI characteristics, it would be possible to effectively utilize the huge conventional library of LSI production technology, so no new development costs would be required. Simultaneously, higher integration could be realized by the two-story structure of an MEMS provided on top of LSI interconnects. Overall higher functions could be expected to be realized as LSI devices with built-in MEMS's.
As explained above, conventional MEMS's are complicated in structure and cannot be reduced in size, so have the problems that they are slow in operating speeds, troublesome in production processes, and unavoidably high in costs and, further, are low in fatigue strength of the moving parts, so are low in reliability.
Note that Japanese Patent Publication (A) No. 2002-526354 proposes a process for production of carbon nanotubes as mechanical elements of MEMS devices.
However, this only shows using a carbon nanotube to form a probe with a cantilever front end utilizing the microsize of the carbon nanotube. It does not suggest at all a movable device provided with an electrode structure or a moving part comprised of a carbon nanotube.
Further, Japanese Patent Publication (A) No. 2005-520450 proposes a micromirror package using an MEMS. A MEMS mirror facing a pad on a semiconductor chip is disclosed, but this is just a simple improvement of a package. A mirror device using a carbon nanotube for the moving part of an MEMS mirror is not suggested at all.